This invention relates to grates and, more particularly, to moving grates which transport refuse or other solid fuels through a combustion chamber in a furnace.
Step grates have been used for many years, as have reciprocating grates, to convey refuse through firing chambers. In reciprocating grates, alternating grate steps consisting of ten (for example) horizontally arrayed grate bars move horizontally to push refuse through the chamber. The refuse is oxidized during its transportation through the chamber and massive amounts of heat are produced. The oxidation, or more specifically, the combustion, is regulated by the quantity of oxygen (generally in the form of air) permitted in the chamber. To aid and regulate combustion, grate manufacturers have provided air vents in the grates. The air vents have also served to cool the grates. This is necessary because the grates can be exposed to direct flames and heat since they support the fuel bed.
Grate manufacturers have attempted to avoid two principle problems, namely, grate corrosion and incomplete combustion. Corrosion of the grate surface is caused principally by exposure of the grate bars to the excessive heat and flame emitted during the oxidation of combustible material during the operation of the furnace. Partial burning of combustible matter is due to loss of the material during its transportation because of openings between grates, either between adjacent grate bars of each grate step or successive steps. This can be a particularly serious problem in the case of certain types of refuse or refuse containing plastics which, if not properly burned, can produce pollutants such as dioxin.
Conventional grates do not satisfactorily avoid these problems. Typical grates contain air vents to cool each grate step and to facilitate combustion by providing the necessary amounts of oxygen at a particular location (i.e., to provide the primary air supply for the combustion). However, these air vents are subject to being clogged with partially burned material, dust or ashes and they can become difficult, if not impossible, to clean. Furthermore, cleaning time cuts into the time during which the furnace can operate and increases the costs of operation. Clogged air vents hamper the circulation of oxygen and therefore decrease the efficiency of the combustion. The cooling action of the air is also lessened and, as a result, the grate reaches temperatures which more readily corrode its surfaces. Conversely, when air coming from the vents produces combination flames, hot air pockets can be formed on the grate surface, causing substantial and serious corrosion. This becomes particularly serious when the air vents are positioned on the lower portion of the front surface of the step because developing flames go up and corrode the surface of the step causing the irregular enlargement of the air vents which can eliminate entirely the front of the grate bar. This allows a high volume of ashes, dust, and unburned or partially burned material to fall beneath the grate, which is particularly serious in the case of polluting refuse.